This invention relates to the preparation of ceramic materials or articles by the pyrolysis of mixtures of vinyl- or allyl-containing preceramic silazane polymers, mercapto compounds, and photoinitiators wherein the mixtures are rendered infusible prior to pyrolysis by ultraviolet (UV) irradiation. This method is especially suited for the preparation of ceramic fibers.
Ceramic materials have been prepared by the pyrolysis of various preceramic polymers in the prior art. Gaul in U.S. Pat. No. 4,312,970 (issued Jan. 26, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting organochlorosilanes and disilazanes. The preceramic silazane polymers were pyrolyzed in an inert atmosphere without any separate treatment to first render the preceramic silazane polymer infusible.
Gaul in U.S. Pat. 4,340,619 (issued July 20, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting chlorine-containing disilanes and disilazanes. Fibers prepared from such preceramic silazane polymers were given a "mild heat treatment" in air before pyrolysis but there is no teaching that such a treatment rendered the fibers infusible.
Cannady in U.S. Pat. No. 4,540,803 (issued Sept. 10, 1985) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting trichlorosilane and disilazane. The preceramic silazane polymers were not rendered infusible prior to pyrolysis to form ceramic materials.
Haluska in U.S. Pat. No. 4,546,163 (issued Oct. 8, 1985) prepared vinyl-containing polysilanes of the average formula EQU [R".sub.2 Si][R"Si][R'".sub.d (CH.sub.2 .dbd.CH)Si]
where R" was an alkyl radical containing 1 to 4 carbon atoms, R'" was an alkyl radical containing 1 to 4 carbon atoms, a vinyl radical or a phenyl radical, d was 1 or 2 and where the polysilane contained 0 to 60 mole percent [R".sub.2 Si} units, 30 to 99.5 mole percent [R"Si] units, and 0.5 to 15 mole percent [R'".sub.d (CH.sub.2 .dbd.CH)Si] units. Shaped articles prepared from such polysilanes could be rendered infusible prior to pyrolysis by UV treatment. Neither mercapto compounds, photoinitiators, or preceramic silazane polymers were employed. Typical fibers required UV exposure times of 0.5 to 20 minutes at a dosage of about 100 mJ/cm.sup.2.
West et al. in Polym. Prepr., 25, 4(1984) disclosed the preparation of polysilanes of general formula EQU [CH.sub.3 (CH.sub.2 .dbd.CHCH.sub.2)Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of allylmethyldichlorosilane and methylphenyldichlorosilane. These allyl-containing polysilanes were rapidly gelled by irradiation with UV light. Neither mercapto compounds, photoinitiators, or preceramic silazane polymers were employed.
Bartos et al. in U.S. patent application Ser. No. 748,109, filed June 24, 1985, now abandoned, obtained infusible preceramic silazane polymers by treatment of the preceramic silazane polymer with steam or a steam and oxygen mixture.
Lipowitz, in a copending U.S. patent application entitled "Infusible Preceramic Polymers via Plasma Treatment", obtained infusible preceramic polymers by treatment of preceramic polymers with a plasma energy source. Both vinyl-containing and vinyl free preceramic polymers were rendered infusible by this technique.
What has been newly discovered is a method of rendering preceramic silazane polymers infusible prior to pyrolysis. This method represents a significant advance in the art of preparing ceramic materials or articles, especially in the art of preparing ceramic fibers.